Ion beam implanters are used to implant or “dope” silicon wafers with impurities to produce n or p type doped regions on the wafers. The n and p type material regions are utilized in the production of semiconductor integrated circuits. Implanting ions generated from source materials such as antimony, arsenic or phosphorus results in n type material. If p type material is desired, ions generated with source materials such as boron, gallium or indium are typically used.
The ion beam implanter includes an ion source for generating positively charged ions from ionizable source materials. The generated ions are formed into a beam an accelerated along a predetermined beam path to an implantation chamber having a target wafer disposed therewithin. The beam is formed and shaped by apparatus located along the beam path en route to the implantation chamber. When operating the implanter, the interior region must be evacuated to reduce the probability of ions being deflected from the predetermined beam path as a result of collisions with air molecules.
As disclosed in Hsu, U.S. Pat. No. 5,886,356, issued on Mar. 23, 1999 which is herein incorporated by reference, the ion source produces a high-density of ions from which the implanter extracts a focused beam of ions and transports the ions to the target wafer.
During ion implantation a surface, such as a target wafer, is uniformly irradiated by a beam of ions or molecules, of a specific species and prescribed energy. The size of the wafer or substrate (e.g. 8 inches or greater) is typically much larger than the cross-section of the irradiating beam which deposits on the wafer as a spot or “ribbon” of about 1 inch. Commonly, in high current machines, the required uniform irradiance is achieved by moving the wafer through the beam.
In a conventional ion implanter 10 such as that shown in FIG. 1, an ion beam 12 is emitted from an ion source 14 and passed through an analyzing magnet 16 having at least one magnetic coil to remove undesired types of ions.
Benveniste, U.S. Pat. No. 5,554,857, issued on Sep. 10, 1996 discloses a method and apparatus for ion beam formation in an ion implanter.
As disclosed in Benveniste, which is herein incorporated by reference, the mass analyzing magnet 16 is positioned along a beam path between the ion source 14 and the implantation chamber deflects ions through controlled arcuate paths to filter ions from the ion beam while allowing certain other ions to enter the ion implantation chamber.
The ion beam current flows through the analyzing magnet coil and generates a magnetic field having a magnetic flux density. Because the ion beam current is proportionate to the magnetic flux density of the magnetic field in the analyzer magnet, a change in beam current will also affect the flux density of the magnetic field of the analyzer magnet. Ions are flow through the analyzing magnet in a circular path in accordance with a right hand rule that is well known in the electromagnetic arts.
Thus, ions having identical energies but different masses experience a different magnetic force as they pass through the magnetic field due to their differing masses thereby altering their pathways. As a result, only those desired ions of a particular atomic mass unit (AMU) are allowed to pass through a pre-positioned orifice in the analyzing magnet 16.
After passing through the mass analyzing magnet the ion beam is accelerated to a desired energy by an accelerator 18. Negative ions are changed into positive ions by a charge exchange process involving collisions with a chemically inert gas such as argon. The positive ions then pass through a post-analyzing magnet (not shown), and a pair of vertical and horizontal scanners 20, 22 finally reach a target wafer 24 where they impact the wafer 24 and are implanted.
However, operation of an ion implanter results in the production of certain contaminant materials. These contaminant materials adhere to surfaces of the implanter beam forming and shaping structure adjacent the ion beam path and also on the surface of the wafer support facing the ion beam. Contaminant materials include undesirable species of ions generated in the ion source, that is, ions having the same atomic mass.
How the AMU calibration device on an ion implanter is calibrated will influence the contamination of ions implanted into a wafer. A calibration error can cause the magnetic coil disposed within the analyzer magnet to select an incorrect ion to implant. An incorrectly implanted ion can cause wafer damage due to scraping of improperly doped wafers.
Typically, a medium current ion implanter is capable of controlling implantation of an ion within +−1 AMU of an AMU of a desired dopant ion. However, a device providing an AMU interlock of less than +−1 is not available in most medium current ion implanter.
A default AMU interlock of +−1 (AMU) can cause pollution when implanting various ions such as a P31 ion. For example, a BF30 ion having an AMU (30), wherein the AMU (30)=B(11) and F(19), within 1 AMU of the P31 ion may be implanted when the P31 ion is being implanted. The presence of the BF30 ion can occur when gas piping leaks during implantation of a P31 ion. Thus, BF30 is a pollution source.
As shown in FIG. 2, a normal phosphorus spectrum has a strongest ion beam current at a P31 ion having an AMU of 31. However, if a contaminate such as BF30 having an AMU within +−1 AMU of the P30 ion is present, then both of the ions may be implanted.
FIG. 3 illustrates a relationship between beam current on the Y-axis and AMU on the X-axis for the ions P31 and BF30. The desired ion to be implanted is P31, having an AMU of 31 has the strongest beam current the closer the P31 ion is to the correct AMU of 31. However, if a contaminate ion BF30, having an AMU of 30 is also introduced within the analyzer magnet, the beam current is also strong at the AMU of 30 for the BF30 ion. Thus, with such a strong beam current for a contaminate ion, it is possible to implant both the BF30 and the P31 using the implanter used in the prior art having an AMU interlock of +−1 AMU.
Because the magnetic field and magnetic field flux density of the AMU analyzing magnet 16 is directly proportional to the beam current, an adverse change in beam current will also adversely affect the strength of the magnetic field, thus allowing for contamination of undesirable ions.
However, because the AMU of the ions in a traditional implanter may vary by +−1 AMU, it is possible to implant contaminate ions that have a slightly different AMU, within +−1 AMU, than a desired ion to be implanted.
Thus, an object of the present invention to provide a tool to make sure that the implanting ion is the desired ion to be implanted.
Thus, the present invention determines if a desired ion having a fixed AMU is being implanted.